Moisture detecting apparatus



NOV- 6, 1951 o. c. HAGAN 2,574,261

MOISTURE DETECTING APPARATUS med July 5, 1949 I8 fw 12,6*

z5 26 27 BY M Wb ATTORNEYS Patented Nov. 6, 1951 MOISTURE DETECTING APPARATUS Owen C. Hagan, Stockton, Calif., assignor to California Cedar Products Company, Stockton, Calif., a corporation of California Application July 5, 1949, Serial No. 102,999

8 Claims.

This invention relates to, and it is an object to provide, a novel electronic apparatus and method for accurately detecting or ascertaining the moisture content of material, especially fibrous material such as wood or paper, in sheet Vor flat form. For example, the invention is extremely useful in the detection of moisture content of wooden pencil slats, as it is requisite that the moisture content thereof be closely controlled. Y

Another object of the present invention is to provide a moisture detecting apparatus and method wherein the moisture content of material being tested is determined from the comparative dielectric constants of said material when engaged on opposite sides by electrode plates of a condenser assembly and air in a gap between the electrode plates of another condenser assembly; the differences in said dielectric constants being resolved in terms of moisture content through the medium of a novel balancing or bridge circuit in which the condenser assemblies are interposed.

A further object of this invention is to provide an apparatus and method, as in the preceding paragraph, wherein the electrode plates of the material engaging condenser assembly are vari# able in spacing to accommodate material oi dif-` ferent thickness engaged therebetween; the other or air gap condenser assembly being mounted in a manner so that the gap of its electrode plates automatically and correspondingly varies in direct response to variance in the electrode plate spacing of said material' engaging condenser assembly. Thus, the comparative values of the respective dielectric constants are not altered upon testing of material of diierent thickness; diierential spacing of the gaps of the condenser assemblies is avoided; and as a result a balancing circuit including said condenser assemblies can be used effectively to determine moisture content.

An additional object of the present invention is to provide a moisture detecting apparatus and method, as above, wherein the balancing circuit includes variable condensers adiustable to place the circuit in balance; said condensers being ganged and simultaneously adjustable by a common dial scaled in terms of moisture content.

It is also an object of the invention to provide a moisture detecting apparatus and method which is relatively simple in structure, circuiting, and operation.

A still further object of the invention is to provide a moisture detecting apparatus and method which is practical, reliable, and exceedingly effective for the purpose for which it is designed.

These objects are accomplished by means of such structure and relative arrangement of parts as will fully appear by a perusal of the following specification and claims.

In the drawings:

Fig. 1 is a diagram of a preferred form of the circuit which is employed in the practice of the invention. ,K

Fig. 2 is a side elevation of the material engaging unit, as in use.

Fig. 3 is an end view of the same.

Referring now more particularly to the characters of reference on the drawings, and at present to Fig. l, the circuit comprises a radio-frequency oscillator, indicated generally at I; such oscillator being of generally standard construction and having a pre-selected frequency, dependent on the material to be tested. Fo:v wooden pencil slats a frequency of approximately 200 kilocycles is quite satisfactory. The frequency should be selected, however, to favor the particular material being tested, as the dielectric constant and the power factor of materials do vary with frequency.

One input lead 2 extends from the oscillator I, and is branched, as at 3 and 4; each such branch having a variable condenser 5 interposed therein. The variable condensers 5 are banked with the interconnection such that when one of said condensers is adjusted to increase its capacity, the capacity of the other condenser is decreased, and vice versa.

The banked variable condensers 5 are manually controlled by a dial 6, calibrated, as at 1, to read on a pointer 8 in terms or percentage of moisture content of material tested.

Beyond the variable condensers 5 the branches 3 and 4 of lead 2 connect to the output terminals 9 of the circuit.

The other input lead, which is indicated at I0, extends from the oscillator I to connection in parallel with four variable gap condensers disposed in a transverse row; the intermediate, material engaging pair of said condensers being indicated at II, and the other, air gap pair of said condensers-disposedat the ends of such row-being indicated at I2. The pair of material engaging condensers II are disposed in relatively close side by side relation.

The input lead I0 is branched, as shown, with the branches connected in parallel to the fixed electrode plates I3 and I4 of the variable gap condensers I I and I2, respectively.

The variable gap condensers II and I2 include movable electrode plates. indicated at I5 accessi..-

are connected in parallel by a branched circuit I1 to one of the output terminals The movable electrode plates I6 of the air gap condensers I2 are connected in parallel to the other output terminal by a branched lead I8.

A cross lead I9 extends between the output terminals 9 and has a crystal detector 20 interposed therein; such crystal detector being a germanium diode in the present embodiment.

Output leads 2I extend from the output terminals 9 to opposite sides of a volt meter 22, which volt meter is of a type lsensitive to the input frequency.

In order to prevent the meter 22. from shortcircuiting the radio frequency, chokes 23 are interposed in each of the output leads 2I; such chokes here being resistors.

The transverse row of variable gap condensers II and I2 are included in a material engaging unit, indicated generally at 24; such unit being adapted to receive the material to be tested, and comprising the following, as shown in Figs. 2 and 3;

The numeral 25 indicates a base, on which a horizontal, transversely elongated block 26 is afiixed by means including end legs 21; such block being of dielectric material such as Lucite.

In juxtaposition above the block 26, the material engaging unit 24 includes an upper, horizontal, transversely elongated block 28 also of dielectric material; the blocks 26 and 28 having adjacent faces which are at and normally parallel.

The upper block 28 is adapted for free vertical floating motion relative to the lower or xed block 26; such motion being guided by a plurality of spaced, opstanding guide pins 29 on the base 25 at each end portion of the assembly of said blocks. The guide pins 29 are arranged so that certain thereof bear against the ends, while others bear against the side edges of the upper block 28, whereby to limit longitudinal as well as lateral motion of said upper block without limiting its vertical free play. Said ends and side edges of the upper block 28 are rounded, as shown, to assure against binding on the guide pins 29.

The fixed electrode plates I3 and I4 of the variable gap condensers II and I2 are secured flat or horizontally to the top surface of the dielectric bottom block 26; the branched input lead Ill connecting to said fixed electrode plates I3 and I4 .by means of terminal pins 30.

The movable electrode plates I5 and I6 of the variable gap condensers II and I2 are secured flat or horizontally against the bottom of the upper block 28 in matching relation to the corresponding xed electrode plates I3 and I4, respectively. The branched leads I'I and I8 are connected to the electrode plates I5 and I6, respectively, by means including terminal pins 3| and 32.

The xed electrode plates I3 and I4. and the corresponding movable electrode plates I5 and I 6, are of like size and configuration, so that with the hypothetical situation of equal air gap between all of the condensers II and I2, said condensers would have like capacity, and balance would exist in the circuit. In other words, the total of the capacity of the two condensers II would be the total of the capacity of the two condensers I2.

The above described circuit and apparatus is l employed to test the moisture content of material by the following method;

By way of example, a wooden pencil Slat 33, to be tested for moisture content, is placed between the lower block 2S and upper block 28 in direct engagement between the pair of material engaging condensers II; the electrode plates I3 and I5 of said condensers bearing on opposite sides of the pencil slat.

With the pencil slat so engaged, there is an automatic adjustment of the air gap of condensers I2 to a distance equal to the thickness of said slat.

An important feature of the invention is the fact that the gap of condensers I2 always remains equal to the gap of the condensers II, regardless of the thickness of the material being tested. As a result the circuit is not thrown out of balance merely by differential gaps between the condensers II and I2.

With the pencil slat 33 in the material engaging unit 24, as above, the dielectric constant of said pencil slat is much greater than the known dielectric constant of the air gap of condensers I2; air having a dielectric constant of one and wood--for instance-having a dielectric constant of three to seven, and the moisture having a dielectric constant of approximately eighty. Therefore, the condensers Il have a much higher capacity than the condensers I2, which only have air as a dielectric. The consequence is that the circuit becomes unbalanced, which unbalance is reflected on the meter 22 relative to a zero or minimum point. Thereafter, the dial 6 is manipulated to actuate the banked but opposed variable condensers 5 until the latter re-balance the circuit, which re-balance will be indicated by the meter 22 reading at zero or said minimum point.

The extent of 'manipulation of the banked variable condensers 5 required to balance the circuit is resolved into percentage of moisture by the calibrations I on the dial 5; the latter reading on the pointer 8.

In short, the described balancing or bridge circuit is operative, electronically, to make an accurate comparison between the dielectric constant of the pencil slat 33 and the known dielectric constant of an air gap equal in length to the thickness of said slat; the comparative dilerential then being employed, by precalculation, to indicate moisture content.

The described apparatus and method functions electively and accurately to the determination of the moisture content of material being tested.

As the upper block 28 is free floating, and guided in non-binding relation, the electrode plates I3 and I5 may rest flush on the surfaces of the material, and if such material should have a slight lateral taper, the decrease in the air gap of one condenser I2 would be automatically and effectively compensated by the increase in the air'gap of the other condenser I2.

The crystal detector 20 is employed in the circuit in the manner described to serve as a rectifier` for the radio frequency current impressed on the circuit, and to the end that the feed in the output leads 2| be D. C.

If desired, the material engaging surfaces of 4 the electrodes I3 and I5 may be insulated with an extremely thin coat or lm of high dielectric strength material to eliminate any tendency for a very wet material to short between said electrode plates.

A valuable feature of the invention is that the stability of the circuit is very high, and the calibration quite permanent, because the frequency of the oscillator and the sensitivity of the meter have negligible effect upon the test; the prime controlling factor being the automatic, like spacing between the electrode plates of all of the condensers I I and l2.

While the invention is described above asV used for testing pencil slats for moisture content, such invention is not so limited, and may be adapted to test many other types of materials.

From the foregoing description it will be readily seen that there has been produced such an apparatus and method as substantially fulfills the objects of the invention as set forth herein.

While this apparatus and method is herein set forth in its present and preferred form, still in practice such deviations therefrom may be resorted to as do not form a departure from the spirit of the invention, as dened by the appended claims.

Having thus described the invention, the following is claimed as new and useful, and upon which Letters Patent are desired:

1. Apparatus for testing material for moisture content, comprising a variable gap condenser having electrode plates between which the material is adapted to engage, another variable gap condenser having electrode plates deilning an air gap therebetween, means mounting the condensers for corresponding adjustment of the gaps thereof, and a. circuit in which the condensers are interposed operative to convert differential in capacities oi said condensers, resulting from engagement of material in said first named condenser, into terms of moisture content; said mounting means including a fixed member and a movable member in juxtaposition, the separate electrode plates of each of the condensers being mounted in adjacent faces of corresponding members in cooperative relation.

2. Apparatus, as in claim 1, including means to guide motion of the movable member relative to the fixed member.

3. Apparatus for testing material for moisture content, comprising a variable gap condenser having electrode plates between which the material is adapted to engage, another variable gap condenser having electrode plates dening an air gap therebetween, means mounting the condensers for corresponding adjustment of the gaps thereof, and a circuit in which the condensers are interposed operative to convert diiferential in capacities of said condensers, resulting from en' gagement of material in said first named condenser, into terms of moisture content; there being balancing means in the circuit, and said balancing means including a movable member, movement of the latter being resolved in termsv of moisture content of the material; the circuiti." being in the form of a bridge, the variable gap)- condensers being interposed in separate leads on one side of the output, and said balancing means being separate variable condensers interposed in separate leads on the opposite side of the output; said separate variable condensers being banked in opposition, and actuated simultaneously by said movable member.

55 Number 4. Apparatus, as in claim 3. in which said movable member is scaled 1n terms of moisture content and said scale reads upon a iixed point upon movement of said member.

5. Apparatus for testing material for moisture content, comprising a row of spaced, variable gap condensers each having cooperating electrode plates, guided relatively separable members, corresponding electrodes being mounted on said members whereby upon separation of the latter 'from engagement of material therein; the circuit including balancing means having 'a movable member whose movement is resolved in terms of moisture content of said material.

6. Apparatus for testing material lfor moisture content, comprising a rst variable gap condenser having electrode plates between which the material is moved, at least one of said plates being movable relative to the other in response to varying thicknesses of material moved'between the "plates, a second variable gap condenser having spaced electrode plates to define an air gap therebetween, at least one of said last named plates being movable relative to the other, and means connecting the movable plates of the respective condensers in rigid relation with each other so that the movable plate of the second condenser will move in response to the movement of the movable plate of the first condenser and to the same extent whereby the gaps between the plates of the respective condensers will remain constant through all movements of such movable plates; and a circuit in which both condensers are interposed and which circuit is operative to convert into terms of moisture content the differential in capacities of the condensers resulting from movement of material through the plates of the iirst condenser.

OWEN C. HAGAN.

REFERENCES C ITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Name Date Clark Sept. 20, 1932 Walter Dec. 11, 1934 Wilcox Jan. 14, 1941 Storm Dec. 2, 1947 Joy July 25, 1950 FOREIGN PATENTS Number 

